“She is horrified about Spencer being Pippa’s brother-in-law,” a source told Life & Style, adding that the 35-year-old fears he’ll spill big family secrets in an effort to become more famous.

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The 27-year-old playboy's past certainly doesn't help. He claims to have bedded over 1,000 women and was kicked off I'm a Celebrity...Get Me Out of Here! for steroid addiction.

"Spencer has slept his way around London and left a lengthy trail of angry women in his wake," a different insider revealed. "Kate's concerned. The last thing she wants is for William's family to turn against her because of Spencer's actions."

Despite her concern, Kate did help her little sister plan her big day.